Long, I am in the same boat as you. For 4 years now I’ve walked every day, and stayed active every day. But that did nothing for maintaining my upper body strength, and I noticed a significant decrease in muscle strength & mass. I joined Planet Fitness only $25 per month. Open 24/7 I believe, and you can bring a friend with you free. I met with a trainer and told him I needed more upper body strength and he showed me which machines to concentrate on and said if I felt I needed personal training from him we could schedule that. All included in membership price. I love the fact there are thousands of them nationwide if I travel somewhere. I’ve only been 2 times but love it, and will at least go 4 day a week.

For upper body, here are exercises with weights: bent over row, chest press, biceps curls, triceps kickback, seated shoulder press and planks (front, left and right sides).

It's important to get the form correct to stay injury free. After some time of trying on my own, I decided to sign up with a trainer to coach me on form and when to increase weight as I was either too aggressive or not aggressive enough with adding weight. 2x a week with one session on my own. Training place is close to where I live -- location, location, location....There are lots of great online options too, so you can be anywhere!